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TREE PRESERVATION BULLETIN NO. 3 



Tree Bracing 



by A. Robert Thompson 

Forester 

National Park Service 



TREE PRESERVATION BULLETIN NO. 3 • REVISED 1959 




UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary 

NATIONAL PARK SERVICE 
Conrad L. Wirth, Director 



For sale by the Superintendent of 
Documents, U.S. Government Printing 
Office, Washington 25, D.C. 
Price 15 cents 



Foreword 



Over 20 years ago the National Park Service was confronted 
with the problem of improving and maintaining in good condi- 
tion thousands of valuable shade, ornamental, or historically sig- 
nificant trees within a variety of areas. In order to guide those 
who were responsible for this work in park areas, a most complete 
and useful series of nine Tree Preservation Bulletins was prepared 
between 1935 and 1940 by the late A. Robert Thompson, forester 
in the Branch of Forestry, National Park Service. The original 
Bulletin No. 3, Tree Bracing, was one of this series. 

Although the bulletins were originally intended for park em- 
ployees, they received wide use of arborists, and this demand 
has continued. They are being reissued from time to time to 
meet this need. 

The revised series will total seven in number. The original 
Bulletins Nos. 1 and 2, which were combined in a single publica- 
tion, are not being reissued because they referred to National Park 
Service practices only. The original Bulletin No. 9 has been re- 
issued as Bulletin No. 1. Transplanting Trees and Other Woody 
Plants. The original Bulletin No. 8 has been reissued as Bulletin 
No. 2, Safety for Tree Workers. Shade Tree Pruning, reissued in 
1955, remains as Bulletin No. 4; General Spraying and Other 
Practices, reissued in 1953, remains as Bulletin No. 6; and Ropes, 
Knots, and Climbing, reissued in 1955, remains as Bulletin No. 7. 
The final publication in the series, Lightning Protection for Trees, 
will be reissued as Bulletin No. 5, retaining its original position. 

This bulletin, Tree Bracing, remains No. 3 in the series. Except 
for very minor revisions, it is essentially unchanged from the orig- 
inal, attesting to Bob Thompson's complete and expert knowledge 
of the subject. 

Conrad L. Wirth, Director. 



in 



Contents 



Introduction 


Page 

1 


Obsolete Bracing Methods 


2 


Modern Objectives and Materials 


3 


Types of Tree Bracing 


4 


Cabling 


5 


Cabling Systems 


5 


Cabling Material Tests 


8 


Cabling Technique 


9 


Lag Installation 


11 


Eyebolt Installation 


12 


Cable Splicing 


14 


Cable Maintenance 


15 


Bolting or Rod Bracing 


15 


Crotch Bracing 


16 


Bracing Rubbing Limbs 


19 


Cavity Bracing 


19 


Rodding Standards 


20 



Introduction 



Tree bracing is one of the most essential phases of an intelligent 
tree preservation program. One of the most common entrance 
places for decay fungi is through wounds caused by splitting of 
limbs. Bracing, therefore, not only prevents the disfigurement 
of trees but it actually prolongs tree life by preventing the forma- 
tion of entrance places for fungus spores. Even after a tree is 
in an advanced stage of decay, bracing may prolong its period 
of usefulness for many years by providing mechanical support. In 
order to achieve its fullest purpose, however, bracing should seek 
to prevent, rather than to correct, damage. 

An otherwise sound tree might require bracing for many rea- 
sons. Typical symptoms indicating a need for bracing might be 
found in (1) split crotches, {2) tight V-shaped crotches, (3) in- 
herent weakness of species, (4) extra heavy foliage growth, (5) 
cut or shallow root systems, (6) decay, (7) proximity to struc- 
tures, (8) removal of nearby trees, (9) borer attack, and (10) 
prevalence of high wind, sleet storms, or other adverse environ- 
mental conditions. 

The second symptom in the previous paragraph should be clearly 
understood. V-crotches are often found in species which have 
multiple or divided trunks. The major limbs may grow so nearly 
parallel to each other that the cambium and bark are pinched 
between them, with resulting death of the constricted tissue. The 
point of contact of the limbs gradually raises as the limbs grow in 
diameter so that a seam of nonconnecting tissue is gradually devel- 
oped between them (fig. 1). This results in a weak structural 
arrangement of the wood elements which may break down under 
strain. The U-shaped crotch, on the other hand, builds up an 
ever-increasing bond between its member limbs so that it rarely 
requires artificial bracing. It is, of course, good judgment to pre- 
vent the formation of V-crotches insofar as this is possible by judi- 



TREE PRESERVATION BULLETIN NO. 3 





V-CROTCH U-CROTCH 

Figure i. — Arrangement of wood elements. 

cious pruning and training when the trees are small. Good nursery 
practice and proper handling of transplated trees will eliminate 
many V-crotches before they become serious. 

Among shade trees which often require bracing are maple, elm, 
willow, hickory, yellow-poplar, linden, and ash. Under certain 
conditions almost any species might require bracing in order to 
overcome or correct structural weaknesses, or unfavorable environ- 
mental conditions. In deciding bracing problems it should be 
remembered that it is usually safer to overbrace rather than under- 
brace, both as to number of braces and strength of materials. 



Obsolete 
Bracing Methods 



Early efforts in tree bracing involved expensive equipment and 
made-to-order materials which were difficult and costly to pre- 
pare and install. These obsolete methods included the use of 
long, rigid iron bars placed high in the trees, iron collars placed 
around split crotches, chains to hold limbs in position, and various 
systems of wires and homemade cables (fig. 2). 

This work was not only very expensive, cumbersome, unsightly, 
and difficult to install, but in many cases the results were actually 



TREE BRACING 




Figure 2. — Typical obsolete bracing methods. 

harmful to the trees. The rigid bars tended to resist normal wind 
sway instead of allowing some movement and thus tended to in- 
crease the danger of splitting ; the crotch collars retarded sap move- 
ment and tree growth, thus weakening and sometimes killing the 
tree; the cumbersome chains were noisy, ugly, expensive, difficult 
to install, and inefficient; and the self-made wire cable systems 
were weak and had many other obvious defects. 



Modern Objectives 
and Materials 



The objectives of modern tree bracing are fourfold: First, the 
bracing materials must be inherently strong; second, the installa- 
tion must be correct from an engineering viewpoint; third, the 
installation must injure the tree as little as possible; and fourth, 
the bracing materials must be reasonably inconspicuous in place. 

We are more fortunate today than were our predecessors of 



4 TREE PRESERVATION BULLETIN NO. 3 

50 or even 15 years ago in our choice of stock materials. Whereas 
formerly it was necessary to have a blacksmith prepare the cum- 
bersome bands, rods, and chains, today we can order from the 
nearest supply house such standardized materials as galvanized, 
copper, or copper-covered wire and strand; galvanized lag hooks, 
hook bolts, or eyebolts; galvanized and bronze thimbles, threaded 
galvanized or duralumin screw rod with hexagonal or eye nuts 
to fit; round, oval, or diamond-shaped washers, special tools and 
drills to fit pneumatic and electric power units, etc. 

These materials may be used in various combinations for such 
purposes as crown cabling, guying, crotch bracing, cavity bracing, 
and the bracing of intersecting limbs. While bracing problems 
will always tax the ingenuity of the tree worker, certain principles 
and practices have been fairly well standardized through labora- 
tory tests and field experience. 



Types of 
Tree Bracing 



Modern arborists usually consider tree bracing under two major 
divisions: Cabling, and rod bracing. The former term is usually 
applied to flexible cables installed high up in trees to take a por- 
tion of the loads off structurally weak crotches. Under the general 
heading of cabling, we can properly consider also the guying of 
transplanted trees. 

The term "rod bracing" usually is applied to bolts or threaded 
rod used for rigid bracing of weak or split crotches, for sewing up 
long splits in trunks or limbs, for holding rubbing limbs together 
or apart, and for various types of cavity bracing. It is frequently 
necessary to resort to both types of bracing in the same tree, but 
for the sake of clarity we shall consider them as separate subjects. 



TREE BRACING 



Cabling 



Flexible cable may be used in bracing trees to strengthen weak 
crotches, to brace trees together, or to support a transplanted tree 
until the root system is reestablished. As a general rule, cables 
should be placed as high in the tree as practicable so as to take 
maximum advantage of the law of the lever and fulcrum and also 
in order to use as small size and as inexpensive material as prac- 
ticable to achieve maximum efficiency. It is usually possible to 
place a cable about two-thirds of the distance from the crotch to 
the branch tips. 

Cabling Systems 

The simplest type of a cable brace is a single line of cable placed 
between two limbs arising from a single crotch.. When several 
limbs in the same tree are to be braced or when trees are to be 
cabled together, the problem becomes more complex. 

Several general systems of applying a multiple set of cables 
are in use. Some of these are of questionable merit from an engi- 
neering viewpoint. Major systems may be classed as box or 
rotary, hub and spoke, multiple direct, and triangular. These 
systems are illustrated in plate I, where it should be noted that 
the same organization of limbs has been used to illustrate each 
multiple system. 

The triangular system, since it involves the best features of other 
systems, has been found to be most efficient for general use. It 
is realized that every application of a cabling system presents in- 
dividual problems and that it is not always possible or practicable 
to use the triangular system. In such cases deviations will suggest 
themselves. 

It is thought well to explain briefly the various systems mentioned 
previously and which are shown on the chart. 

Simple direct. This system involves the installation of a single 
cable directly supporting two limbs arising from a single weak or 
split crotch. 



5 TREE PRESERVATION BULLETIN NO. 3 

Triangular. The triangular system combines the best features 
of all in that it provides direct support for weak or split crotches 
and provides lateral supports which minimize twisting and act as 
supplements to the direct supports (fig. 3) . 

Box or rotary. This is based on a rotary system of cables which 
are attached to the main limbs of a multibranched tree. Each 
connected limb is given lateral support, but no direct support of 
weak or split crotches is provided. The box system permits maxi- 
mum crown movement. 




Figure j. — Triangular cabling system. 



TREE BRACING 




PLATE I 

CABLING SYSTEMS 



FIG. 4 
DIRECT 



FIG. 5 
TRIANGULAR 




FIG. 7 
HUB AND SPOKE 




FIG. 6 
BOX OR ROTARY 



FIG. 9 
GUYING 




<i 



FIG. 10 
LAG HOOK 




FIG. 12 
DROP- FORGED EYE 




FIG. II 
BENT EYEBOL 





FIG. 15 
7-WIRE STRAND 



FIG. 16 
3-WIRE STRAND 



Hub and spoke. This system is based on a group of cables which 
radiate from a ring in the center of a series of branches. It is 
usually of minor value since no lateral support and little direct 
support is provided. 

Multiple direct. This type of cabling is good as far as it goes, 
but lateral as well as direct support should be provided. 

508289 0—59 2 



8 TREE PRESERVATION BULLETIN NO. 3 

Guying. Guying involves the installation of three or more guy 
wires to prevent a transplanted tree from being blown over. Wires 
may be run through short sections of hose to prevent rubbing and 
constriction where they pass around the trunk, but better practice 
is to connect wires to lag screws which are inserted in the tree. 
Grounded ends of guys may be attached to logs, rocks, or steel 
anchors sunk in the earth as deadmen, or they may be attached 
to heavy stakes. Guys may be tightened by twisting if several 
single wires are used, or by turnbuckles if wire strand is used. 
To be effective, they must have regular maintenance. 

Intertree bracing. This is a practice of some value when pro- 
tecting trees have been removed or where anchor roots have been 
cut or exposed. Cables should be placed comparatively high in 
the trees, and support should be given from several directions, 
especially from the direction of prevailing winds. Generally this 
practice should be avoided and used only in exceptional cases. 

Cabling Material Tests 

In order that bracing standards might be based upon scientific 
knowledge, the National Park Service, in 1935, obtained a large 
quantity of practically all materials and sizes used in tree bracing 
and prepared several hundred units, which were tested through 
the cooperation of the Bureau of Standards. The objectives and 
results may be briefly stated as follows : 

1. Objective. To determine the most efficient size hole to 
drill in several species of live wood for all commercial sizes and 
lengths of lags in order to achieve maximum holding power. 

Result. Lead holes one-sixteenth of an inch less in diameter 
than lags were determined to be generally satisfactory for all com- 
mercially available lag hooks. 

2. Objective. To determine the tensile strength of all sizes of 
lag hooks, hook bolts, open eyebolts, eye screws, drop-forged eye- 
bolts, 7-wire galvanized strand, and so-called Amon nuts. 

Result. The relative strengths of materials tested are given 
in table 1 . 

3. Objective. To determine the most efficient method of mak- 
ing a cable eye splice. 



TREE BRACING * 

Result. A wrapped eye splice with a thimble was shown to be 
as strong as the strand itself, two wraps of each wire being ample 
for maximum strength. Previous tests have shown that wire 
rope clips are only 13 to 15 percent as efficient as the strand. 

4. Objective. To determine the rate of stretch of 7-wire gal- 
vanized strand. 

Result. Galvanized strand stretches approximately 1.1 inches 
per square foot before reaching a breaking load. 

As a result of these tests we now know the proper size holes to 
drill for lags, the tensile strength of various sizes and types of lag 
hooks, eyebolts, 7-wire strand, and thimbles, and the most efficient 
type of cable eye splice. Knowing these facts we were able to 
devise an efficient technique and proper combinations of materials 
for given tensile loads. 

These standards for cable bracing are shown in table 1 . 

Cabling Technique 

In deciding the location of cables, several factors must be kept 
in mind. Cables should be so placed that they will not rub against 
limbs and cause abrasions. Lags are as satisfactory as bolts in 
bracing sound hardwood branches, but eyebolts are safer for use 
in bracing softwoods or hardwoods having some decay. Lags 
should never be placed in the direct line of intersection of crotches, 
and if it is necessary to install a lag or bolt near a crotch it is better 
to stay from 6 to 12 inches above or below the crotch intersection. 
A cable usually should be so placed that the cable anchors (lags 
or bolts) on each limb are equidistant from the crotch. In a tri- 
angular system the purpose is to build up a triangle or a series of 
triangles between limbs so that each limb being braced is benefited 
from at least two directions. Only one cable should be attached 
to a lag, and lags should be at least 1 foot apart. 

After a decision has been reached as to the general location of 
required cables, the comparative tensile load to be placed upon 
each cable must be judged. This is not simple, and considerable 
experience is needed to be able to judge fairly the size of cable 
needed in each location. Thought must be given to species, gen- 
eral condition of the tree, exposure to winds, presence or absence 
of decay, whether cavity work has been done or will be done in 



10 



TREE PRESERVATION BULLETIN NO. 3 



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TREE BRACING 11 

the tree, condition and shape of crotches, amount of foliage, size 
of limbs, etc. 

The obvious difficulty in applying these factors is the human 
element which must be depended upon to estimate the maximum 
load which will be supported by the cabling system. As far as 
is known, no sure formula has been determined for estimating 
this load. Attempts have been made to use formulas based upon 
dead weight of limbs, wind stress on tree crowns, distance of sup- 
port from crotch, diameters of limbs, etc., but none of these will 
apply generally, owing to the vagaries of growth. An examination 
of several hundred galvanized cable systems which had been in- 
stalled by thoroughly experienced men led to the conclusion that 
a |4-inch cable or its equivalent is a safe size for limbs up to 6 
inches in diameter at the point of attachment, and a %6-inch 
cable or its equivalent is satisfactory for limbs up to 10 inches in 
diameter at the point of attachment. In most cases the point of 
attachment was about two-thirds the distance from the crotch to 
the end of the limbs. 

A decision having been reached concerning the tensile load to 
be provided for, reference to table 1 will give the proper combina- 
tion of materials for maximum efficiency. 

The most efficient working party for an extensive cabling op- 
eration is usually a 3 -man crew with one man on the ground 
for splicing and the other two in the tree, one man working at each 
end of the cable locations. 

Lag Installation 

As a rule, lags should be inserted in holes which are drilled 
one-sixteenth of an inch smaller in diameter than the lag. The 
hole should be drilled slightly deeper than the length of the lag 
thread so as to prevent splitting, and it should be so drilled that 
the cable and lag form a straight line at the point of attachment. 
Lags should be screwed in up to a point which will just allow 
slipping the cable splices over the hooks. 

After the lags are inserted, the limbs may be drawn together 
slightly by means of a rope slung between the limbs and tightened 
by twisting, a lineman's come-along, a running bowline, a block 
and tackle, or by the use of right- and left-hand lags. This is 
done to avoid slack in the cable which would surely appear after 



12 TREE PRESERVATION BULLETIN NO. 3 

the cable has had a chance to stretch. Our tests have shown that 
7-strand cable achieves a final stretch of about 1.1 inches per 
foot of length before reaching a breaking point. It is important 
that no slack be left in a cable or appear later so as to avoid sudden 
heavy loads placed upon the cable by jerks caused by gusty winds. 

The distance the limbs should be drawn together is dependent 
upon the size of the limbs, the foliage load, and the distance to 
the crotch, and no specific rules can be laid down. Judgment 
must be carefully exercised in each case to avoid immediate or 
later slack. Care must be taken so that the upper foliage is not 
drawn too tightly together. 

With the lags in place, the distance between is carefully meas- 
ured with a tape and the information given to the ground man. 
In the meantime he has prepared an eye splice in the end of the 
cable roll, as described later, and he now kinks the cable at the 
point indicated by the cablers and cuts the cable at a point 10 to 
12 inches below the kink so as to allow material for a splice. After 
the splice is made, the cable with a splice at each end is hauled 
up the tree and hooked over the lags. If the lag has previously 
been turned in so that when the head is at right angles to the di- 
rection of the limb there is just enough space to slip the splice over 
the hook, then the lag should be given a quarter turn so that the 
head is parallel to the direction of the limb. The opening of the 
hook should touch the bark of limb and the splice becomes locked 
in place and cannot jump off the hook. Cables may also be locked 
by inserting a nail in a hole in the hook of the lag if a drilled lag 
is used or one strand of the splice may be left free for wrapping 
around the lag hook. The rope sling or other tightening device 
may then be released and the job is completed. 

Perhaps a word should be added here regarding the superior 
value of copper-covered steel wire and strand as opposed to the 
commoner galvanized strand. It is true that the former material 
is slightly higher in initial cost, but size for size it is considerably 
stronger. It will not rust, and therefore the ultimate cost of the 
bracing operation is reduced. 

Eyebolt Installation 

If eyebolts or hook bolts are used as cable anchors, the general 
cabling procedure is approximately as previously described for 



TREE BRACING 



13 



lag installation. The holes for such bolts should be drilled the 
same diameter as the bolts, however, and it is necessary to provide 
a countersink for the nut and washer. A countersink is prepared 
in the following manner. The exact shape of the washer (round, 
oval, or diamond) is first marked with a chisel on the bark. Then 
this bark and about one-eighth of an inch of wood is excavated 
so that the wound will heal properly (fig. 17) . The cambium edge 







Figure ij. — Countersink,. 

should then be shellacked. The back of the countersink must be 
exactly at right angles to the direction of the bolt so that the 
washer may lay perfectly flat. 

The splicing of the cable is easier done on the ground but may 
be accomplished readily after the bolts are inserted in the holes. 
Use may be made of the threads on the bolt to draw the cable taut 
if the bolts are long enough, although it is usually necessary to 
use some other method of drawing the limbs together also. 



14 



TREE PRESERVATION BULLETIN NO. 3 



It is important that the bolt and cable form a straight line so 
as to minimize movement of the bolt in the hole, which would 
prevent healing. For the same reason and to prevent the cable 
jumping off the hook, it is necessary that the head of a hook bolt 
be drawn snugly against the limb. The eye of the hook or eyebolt 
should, of course, be left parallel to the direction of the limb. 

Before the bolt is thrust into the hole it is well to cover the 
threads with a thick plastic material such as roofing putty or tree 
dressing, in order that water and air may be excluded. The 
entire countersink should be painted with a good tree-wound 
dressing. 

Cable Splicing 

A cable may be attached to a lag or eyebolt by means of an 
eye splice or a cable clamp, such as is used by line companies and 
some tree workers. Tests have shown that a cable clamp has an 
efficiency of only one-seventh of the cable itself, while our tests 
have definitely shown that a properly constructed eye splice squals 
the cable in efficiency. The eye splice is also more desirable than 
the cable clamps because of relative inconspicuousness. 

An eye splice in 7-wire strand is made as follows (fig. 18) : 
First a loop is made by bending the cable about 10 to 12 inches 
from the end, then a thimble, or eye, is inserted in the loop. The 
wires of the 10-inch section are then unwrapped and laid along 





BCD 
Figure 18. — Steps in making an eye splice in j-wire strand. 



TREE BRACING 15 

parallel to the main piece of cable. One strand is selected and 
wrapped with pliers tightly around both the cable and the remain- 
ing six strands. Two wraps are made with this strand, which is 
then cut off. The rest of the strands are then wrapped, one at a 
time, as with the first strand. This results in a tapered eye splice 
which is not only efficient but is inconspicuous. Our tests have 
shown that one wrap of each strand will give maximum efficiency, 
but, since it is practically as easy to make two wraps, this number 
should be used for the sake of a possible margin of safety and for 
the improved appearance. 

An eye splice in copper-covered steel wire or strand is formed 
essentially as described above except that in twisting the individual 
wires a wire server is used instead of pliers, which would scrape 
the copper coating. 

Cable Maintenance 

In practice it has been found that no matter how carefully cable 
systems are installed, it is good judgment to provide periodic in- 
spections in order that slack may be taken out of an occasional 
sagging cable, that rust spots may be touched up, or rusty sections 
replaced, etc. Normal tree growth may cause cable sag, and this 
normal growth may also indicate need for additional cables placed 
higher in the tree. No exact figures are available on the average 
life of a cable, but common sense dictates that each cable should 
be thoroughly checked and replaced if necessary at least every 3 
years. Ordinarily, cables should be checked each time the tree 
is worked and corrections made as necessary. 



Bolting or 
Rod Bracing 



The term "bolting," or "rod bracing," is applied to rigid bracing 
accomplished with the use of prepared lengths of steel rod of 
various diameters which have been threaded with lag thread or 



16 TREE PRESERVATION BULLETIN NO. 3 

machine thread. The common term for such material is screw 
rod, and it may be used with or without nuts and washers on the 
ends of the bolts. 

When nuts and washers are not used, the holding power of the 
screw rod is dependent upon the self-threaded channel made in 
the wood as the rod is screwed in, and hence the hole must be of 
smaller diameter than the rod. When nuts and washers are used 
on the rod ends, it is customary to drill the rod hole the same 
diameter as the rod or larger. Since it is good judgment to use 
all of the holding power available, however, the rod hole should 
normally be drilled smaller than the screw rod to be used, except 
where it is necessary to draw parts of a tree together. 

The screw rod has four major uses in modern practice of tree 
preservation. These major uses may be classified as crotch bracing, 
lip bolting, holding limbs tightly together or apart, and cavity 
bracing (pi. II) . 

Crotch Bracing 

Screw rod is used in modern practice where it is necessary to 
provide a rigid crotch brace. Split crotches usually require such 
provision, and weak but unsplit crotches are often benefited by 
rigid bracing as a supplement to cable bracing. No attempt will 
be made in this bulletin to describe the wound treatment required 
by a split crotch since this matter is discussed in Tree Preservation 
Bulletin No. 4, Shade Tree Pruning. 

Single rod. It is sometimes sufficient in small trees to drill one 
hole through or just above the crotch and insert a piece of screw 
rod so that the two sections of the crotch are securely fastened 
together (fig. 20, pi. II). The problem usually is not so simple, 
however, and large split or weak crotches may require two or more 
lengths of screw rod in order to achieve necessary strength and 
rigidity. 

Parallels. When two rods are used in bracing a single crotch, 
these rods are usually placed parallel to each other and side by 
side, rarely one over the other. These rods are known among 
arborists as "parallels" by reason of their position (fig. 19, pi. II). 
If the rods are to be used simply to maintain the relative position 
of the two limbs being braced, the holes for the rods are drilled 



TREE BRACING 



17 



PLATE H 

ROD BRACING 




HEXAGONAL NUT 



FIG. 24- 
SCREW ROD 




AMON NUT 



SCRFW RQD _ 




FIG. 25 

LIMBS BOLTED TIGHTLY TOGETHER 



FIG. 26 

SEPARATED AND FIXED IN POSITION 



one-sixteenth of an inch less in diameter than the rods so that the 
threads may cut a similarly threaded channel in the wood as the 
rods are screwed in. If the rods have a holding length at each 
end of at least 6 inches of sound wood, this self-threaded channel 
will provide sufficient holding power. But if the wood is even 



18 TREE PRESERVATION BULLETIN NO. 3 

slightly rotted or is apt to become so in the future, or if the wood 
thread is less than 6 inches, it is necessary to cut countersinks (as 
described under "Cabling") and to place nuts and washers on 
each end of each rod. 

Parallel rods rarely should be placed closer than 5 inches or 
farther than 18 inches from each other. A rule of thumb which 
may be used in deciding how to place parallels is based upon the 
diameter of the limbs of the crotch at the point of insertion. In 
using this rule, parallels are separated a distance which is equal 
to one-third to one-half of the diameter of the limbs. Thus paral- 
lels used to brace two 15-inch limbs should be placed from 5 to 
7 J/2 inches apart. 

Safety bolts. Very often it is necessary to increase the safety 
factor of braced crotches over that provided by parallel rods. 
The rigidity should often be increased also. When this additional 
safety factor is required, it is supplied by the installation of an 
additional length of rod from 1 to 3 feet above the parallels in 
a position which is parallel to, and approximately equidistant from, 
each rod (fig. 21, pi. II). The same requirements for hole diam- 
eter apply to safety bolts as have been discussed for parallels, except 
that nuts and washers should always be used on safety bolts as an 
additional factor of safety. 

Lip bolting. The term "lip bolts" may be applied to rods which 
are used to "sew up" a long split in a limb or trunk (fig. 22, pi. II) . 
Lip bolts are often used in cavity bracing but are also very useful 
in providing mechanical support even though no cavity treatment 
is accorded the split area. The same rules for hole size apply to 
lip bolts as were previously mentioned for parallels. Lip bolts 
usually should be placed approximately 12 to 16 inches apart, 
and care should be taken that consecutive holes are not in the same 
direct line of sap flow but are staggered. When lip bolts are in- 
stalled in thin-walled wood, it is sometimes necessary to use nuts 
and washers on each side of the wood wall to provide rigidity and 
to assure that the walls will not be drawn together. 

It must not be inferred by the foregoing paragraphs that bolt- 
ing is necessarily limited to the types described. While the use 
of parallels, safety bolts, and lip bolts will usually provide suffi- 
cient rigid bracing, it is often necessary to add to these bolts by 
supplementary rods. These may be installed as the necessity arises 



TREE BRACING 19 

and in such quantities as experience and good judgment dictate. 
A tightly fitting pipe covering placed over exposed rods will in- 
crease their rigidity to a considerable degree as well as protect 
them from the weather. 

Bracing Rubbing Limbs 

When two limbs develop so that they rub together, it is usually 
desirable to remove one of the offenders. This practice would often 
destroy a very desirable limb, however, and it is frequently con- 
sidered better judgment to resort to bracing. Rubbing limbs may 
be either held tightly together or braced apart. 

If limbs are to be held together and encouraged to form a graft 
(fig. 25, pi. II), the cambium should be traced away from the 
points of contact, and the resulting wounds should be given nec- 
essary treatment; then a hole should be drilled directly through 
the two limbs for the bolt. Usually it is necessary to provide nuts 
and washers to augment the holding power of the thread in the 
wood and, of course, countersinks should be provided for nuts and 
washers when used. 

If limbs are to be held apart, they should be blocked apart tem- 
porarily while a hole is drilled in a direct line through the two 
limbs. As the rod is inserted, either two nuts and two washers or 
a pipe covering should be placed on the rod between the two limbs 
in order to provide a permanent separated position (fig. 26, pi. II) . 
The ends of the bolt should, of course, be provided with nuts and 
washers in the usual manner. 



Cavity Bracing 

Regardless of the type of treatment to be accorded a major tree 
wound, a certain amount of bracing is usually required in an 
attempt to replace artificially the inherent strength of the decayed 
or removed woody tissue and to hold the cavity walls in position. 
Much of the previous discussion concerning crotch bracing, lip 
bolts, etc., may be applied to cavity bracing, but additional strength- 
ening is sometimes indicated. This work may be classed as in- 
ternal bracing. This may take the form of cross bracing or back- 
bone bracing. 



20 TREE PRESERVATION BULLETIN NO. 3 

Cross bracing involves alternate placing of screw rods diag- 
onally across a deep cavity so that in plan the rods form an X 
shape. Such bracing will keep the side walls from spreading and 
will tend to minimize the twisting strains that do so much damage 
to filling materials (fig. 23, pi. II) . 

Backbone bracing is sometimes used in an attempt to reduce 
longitudinal wind sway, but the stresses set up in a large wound 
and the leverage exerted by a tree in a heavy wind are so great 
as to preclude any major benefits from this type of bracing. Its 
use is rarely recommended. Indeed, if a tree is in such a bad 
condition as to indicate the need of extensive internal bracing of 
any kind, it is usually better judgment to replace the tree with 
a healthy one. 

Rodding Standards 

1. Standard screw rods should be cadmium-plated lag-threaded 
rods in $/%-, 3 A~, and 1-inch diameters. Cadmium-plated hexag- 
onal nuts and diamond-shaped cadmium-plated washers should 
be provided for these sizes of rods. 

2. Holes for all rods should be drilled one-sixteenth of an inch 
smaller in diameter than the rods except where it is necessary to 
draw limbs or split trunks together, in which case holes of the 
same diameter as the rods are to be used. 

3. Washers should invariably be countersunk not less than one- 
eighth of an inch below the cambium, in holes of exact shape and 
size of the washers. The floor of such countersinks must be so 
cut that the washers will lay perfectly flat and exactly at right 
angles to the rod. 

4. All rod holes except countersinks should be reamed below the 
cambium so as to avoid springing of cambium and bark. 

5. All rods should be covered with a mastic or tree- wound dress- 
ing as they are being inserted, and all countersinks and reamed 
holes should be painted with approved wound dressing after nuts 
and washers are covered with a metal-preservative paint. 

6. All exposed rods should be covered with tightly fitting pipe 
which should extend the full length of the exposed area so as to 
provide protection and extra stiffness. 



TREE BRACING 21 

7. All exposed rods, pipe nuts, and washers should be painted 
with a metal-preservative paint. 

8. When rods and nuts are to be used for drawing split crotches 
together, it is often necessary to provide two or more washers 
under each nut for additional stiffness. A round washer between 
the diamond washer and the nut will help to keep the diamond 
washer from twisting. 

9. When a rod is to be broken off below the cambium, the rod 
should be cut nearly through before it is inserted in the hole. 
This cut should be approximately l/ 2 inches from the rod end 
so as to allow purchase for a stillson wrench. Care must be taken 
when inserting rod so that wrench marks do not damage threads 
that have to engage wood or nuts. 

10. In drilling for screw rods, it is important to have holes in 
opposite limbs exactly lined up. A long extension bit is very 
useful for this purpose. If such a bit is not available, observers 
from at least two angles should direct the drilling to make sure 
that the holes will be in alinement. 

11. Care must be exercised in tightening nuts on screw rods 
so that the bark and cambium are not injured. A socket wrench 
should be used for this purpose. One end of the thread on the 
rod should be left exposed after tightening the nut, and the rod 
rounded off with a ball peen hammer to prevent the nut from 
working loose. 



U. S. GOVERNMENT PRINTING OFFICE: 1959 O-508289 



UNIVERSITY OF GEORGIA LIBRARIES 



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